1. Field of the Invention
The present invention relates to a switched-capacitor-type stabilized power supply device.
2. Description of the Prior Art
A conventional switched-capacitor-type stabilized power supply device will be described with reference to FIG. 7. An input terminal IN is connected to the positive side of a capacitor C2 and to the input side of a voltage step-up circuit 12. The negative side of the capacitor C2 is grounded.
The voltage step-up circuit 12 is provided with a capacitor C1 and switching devices SW11 to SW14. The node between one end of the switching device SW12 and one end of the switching device SW13 is connected to the input side of the voltage step-up circuit 12. The other end of the switching device SW12 is connected to one end of the switching device SW11, and the other end of the switching device SW11 is connected to the output side of the voltage step-up circuit 12. The other end of the switching device SW13 is connected to one end of the switching device SW14, and the other end of the switching device SW14 is grounded. One end of the capacitor C1 is connected to the node between the switching devices SW11 and SW12, and the other end of the capacitor C1 is connected to the node between the switching devices SW13 and SW14.
The output side of the voltage step-up circuit 12 is connected to one end of a resistor R1, to one end of a capacitor C3, and to an output terminal OUT. The other end of the capacitor C3 is grounded. The other end of the resistor R1 is grounded through a resistor R2.
The node between the resistors R1 and R2 is connected to the non-inverting input terminal of a comparator 3. Connected to the inverting input terminal of the comparator 3 is the positive side of a constant voltage source 4 that output a reference voltage Vref1. The negative side of the constant voltage source 4 is grounded. The output terminal of the comparator 3 is connected to a control circuit 5, which is connected to the control terminals of the switching devices SW11 to SW14. The comparator 3 is of the type that exhibits hysteresis.
Now, the operation of the conventional switched-capacitor-type stabilized power supply device configured as described above will be described. A direct-current power source (not shown) is connected to the input terminal IN so that an input voltage Vin is applied to the input terminal IN. The control circuit 5 turns on and off the switching devices SW11 to SW14 according to the level of the output signal S1 of the comparator 3, which will be described later. The control circuit 5 incorporates an oscillator, and evaluates the level of the output signal S1 of the comparator 3 every period T.
When the output signal S1 of the comparator 3 is at a low level, the control circuit 5 performs alternately, by switching every period T, charge control operation in which it keeps the switching devices SW12 and SW14 on and the switching devices SW11 and SW13 off and discharge control operation in which it keeps the switching devices SW12 and SW14 off and the switching devices SW11 and SW13 on.
On the other hand, when the output signal S1 of the comparator 3 is at a high level, the control circuit 5, rather than switching between the two types of control operation every period T, performs only charge control operation in which it keeps the switching devices SW12 and SW14 on and the switching devices SW11 and SW13 off.
As a result of the control circuit 5 performing charge control operation, the capacitor C1 of the voltage step-up circuit 12 is charged, and its charge voltage reaches Vin. During this charge period, an output current flows from the output terminal OUT to a load (not shown) connected to the output terminal OUT, and therefore the capacitor C3 discharges, and the output voltage Vo lowers.
On the other hand, as a result of the control circuit 5 performing discharge control operation, the negative side of the capacitor C1 is connected to the input terminal IN, and thus the potential at the negative side of the capacitor C1, which was equal to zero when the control circuit 5 was performing charge control operation, becomes equal to Vin. Accordingly, the potential at the positive side of the capacitor C1, which was equal to Vin when the control circuit 5 was performing charge control operation, becomes equal to 2xc3x97Vin. In this way, during the discharge period, a voltage stepped up by a factor of 2 is fed to the capacitor C3, and thus the output voltage Vo increases.
The resistors R1 and R2 serve as a voltage detecting means for detecting the output voltage Vo, outputting a division voltage Va of the output voltage Vo to the comparator 3. The comparator 3 compares the division voltage Va of the output voltage Vo with the reference voltage Vref1 and, when the division voltage Va of the output voltage Vo is higher than or equal to the reference voltage Vref1, turns the output signal S1 to a high level.
Since the comparator 3 is of the type that exhibits hysteresis, once it turns the output signal S1 to a high level, it keeps the output signal S1 at a high level even when the division voltage Va of the output voltage Vo becomes lower than the reference voltage Vref1. When the output voltage Vo becomes so low that the division voltage Va of the output voltage Vo is lower than Vref1xe2x80x2 ( less than Vref1), the comparator 3 turns the output signal S1 from a high level to a low level.
As a result of the operation described above, the division voltage Va of the output voltage Vo is kept in the range from Vref1xe2x80x2 to Vref1 and the output voltage Vo is thereby stabilized within a predetermined range, so that the output voltage Vo is kept substantially equal to the set output voltage Vo*.
In the conventional switched-capacitor-type stabilized power supply device shown in FIG. 7, the voltage step-up circuit 12 employs a 2xc3x97 voltage step-up circuit that steps up the input voltage by a factor of 2. It is possible, however, to realize voltage step-up circuits of various voltage step-up factors, such as 1.5xc3x97 and 3xc3x97, by varying the combination of switching devices and capacitors used in them.
A battery is generally used as a direct-current power source for supplying electric power to a switched-capacitor-type stabilized power supply device. To extend the life of the battery, it is essential that the switched-capacitor-type stabilized power supply device operate stably until the battery voltage falls considerably low, and that it operate with as high power conversion efficiency as possible. In recent years, in particular, switched-capacitor-type stabilized power supply devices have been increasingly used as power sources for driving blue or white LEDs used as backlights for liquid crystal displays incorporated in cellular phones. This trend has been increasing the demand for switched-capacitor-type stabilized power supply devices that permit extended battery lives.
To permit a switched-capacitor-type stabilized power supply device to operate stably until the battery power falls considerably low, it needs to be provided with a voltage step-up circuit with a high voltage step-up factor.
However, inconveniently, increasing the voltage step-up factor of the voltage step-up circuit increases the difference between the voltage stepped-up by the voltage step-up circuit when the battery voltage is still high and the set output voltage Vo*, and thus lowers power conversion efficiency. For example, in the case of the conventional switched-capacitor-type stabilized power supply device having a 2xc3x97 voltage step-up circuit shown in FIG. 7, its power conversion efficiency xcex7[%] is approximated as (100xc3x97Vo)/(2xc3x97Vin), and thus, when, for example, Vo=Vin, the power conversion efficiency xcex7 is 50%. Moreover, where the voltage step-up circuit has a fixed voltage step-up factor n, as in the conventional switched-capacitor-type stabilized power supply device, the switched-capacitor-type stabilized power supply device needs to withstand Vinxc3x97n. Thus, inconveniently, increasing the voltage step-up factor n of the voltage step-up circuit requires designing the switched-capacitor-type stabilized power supply device to withstand an accordingly high voltage.
An object of the present invention is to provide a switched-capacitor-type stabilized power supply device that offers high power conversion efficiency even when the input level to it and/or the output level from it varies greatly.
To achieve the above object, according to the present invention, a switched-capacitor-type stabilized power supply device is provided with: an input terminal to which a direct-current voltage is applied; a plurality of voltage step-up circuits each having a different voltage step-up factor; an output-side capacitor that is charged with the output voltage from the voltage step-up circuits; a voltage detecting circuit for detecting the voltage across the output-side capacitor; a control circuit for turning switching devices on and off according to the voltage detected by the voltage detecting circuit; a switching circuit for connecting and disconnecting the input terminal to and from the voltage step-up circuits; and a switching control circuit for controlling the switching circuit according to the input level to and/or the output level from the switched-capacitor-type stabilized power supply device. Here, the voltage step-up circuits each have a capacitor and a switching device, which is turned on and off by the control circuit, and operate by charging and discharging the capacitor through the switching operation of the switching device so as to step-up the direct-current voltage and output a stepped-up voltage while the capacitor is discharging.